1. Field of the Invention
The present invention relates to a Raman amplifier that can compensate attenuation which a signal suffers when it is transmitted in an optical communication system.
2. Description of the Background Arts
In an optical communication system, a signal lights suffers attenuation when transmitted in an optical transmission line, and consequently its power decreases when it reaches a receiver. If the power of signal lights which have reached a receiver is less than a given value, the performance of normal optical communication cannot be achieved. Therefore, it is necessary to provide an optical amplifier between a transmitter and a receiver in order to compensate such transmission attenuation.
As for such optical amplifier, there are a rare earth element doped optical fiber amplifier using a rare-earth-doped optical fiber for amplification and a Raman amplifier using Raman amplification phenomenon in an optical fiber. The characteristic of the Raman amplifier is that its spectrum band having gain can be shifted by the wavelength of pump light.
In a Wavelength Division Multiplexing (WDM) optical communication system, a Raman amplifier is required to amplify signal lights in a wide spectrum band or a plurality of spectrum bands. For example, in Literature (1): ┌Y. Emori, et al., xe2x80x9c100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodesxe2x80x9d, OFC ""99, PD19 (1999), there is disclosed a Raman amplifier in which an attempt was made to expand the width of the wavelength having gain by introducing pump light of 12 different wavelengths into a single test fiber (i.e., an optical fiber for Raman amplification).
It is an object of the present invention to provide a gain module and a Raman amplifier of low cost and an optical communication system in which a wavelength width having gain is broad.
In order to achieve this object, a gain module according to the present invention is provided with (1) a plurality of optical fibers which differ from each other with respect to the composition of their respective optical regions and through which signal lights are amplified by stimulated Raman scattering and (2) one or more pump light sources which supply each of the optical fibers with pump light for Raman amplification. In this specification, a region (region inside the mode field diameter) in which optical power is equal to or more than exe2x88x922 of the optical power in the core center is called an xe2x80x9coptical region.xe2x80x9d The term xe2x80x9cRaman amplificationxe2x80x9d as used herein means amplifying signal lights by stimulated Raman scattering.
In one embodiment of the present invention, the difference of the Stokes shift of optical fibers may be equal to or more than 200 cmxe2x88x921 or not less than 400 cmxe2x88x921. A plurality of optical fibers for Raman amplification may be connected in series or in parallel. The pump light source may supply pump light of a substantially identical wavelength or pump light of a different wavelength to each of the optical fibers. Also, the pump light source may supply pump light from one common pump light source. The optical region in some of the optical fibers may be doped with GeO2 or P2O5.
An optical communication system according to an embodiment of the present invention is provided with optical transmission lines composed of a plurality of optical fibers installed in station sections the optical regions of which optical fibers differ from each other with respect to their composition and through which signal lights are amplified by stimulated Raman scattering.
An optical communication system according to another embodiment of the present invention is provided with optical transmission lined composed of a plurality of optical fibers being located at a station the optical regions of which optical fibers differ from each other with respect to their composition and through which signal lights are amplified by stimulated Raman scattering.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.